Equalizers are widely used in many electronic devices. For example, FIG. 1 illustrates an optical recording/reproducing apparatus 100. As shown in FIG. 1, the optical recording/reproducing apparatus 100 includes an optical recording medium 101, an optical pick up unit 103, an analog front end (AFE) unit 120, a digital signal processor (DSP) 107, a decoder 109, a host interface (I/F) 111, and a host 113.
The optical recording medium 101 may be any medium capable of storing data and of having data read therefrom. There are various kinds of optical recording media including, for example, compact disc-recordable (CD-R), compact disc-rewritable (CD-RW), digital versatile disc-recordable (DVD-R), and so forth.
The optical pick up unit 103 may include laser diodes for respective wavelengths, a plurality of passive optical devices, a plurality of light receiving devices, and a plurality of passive optical device control operating devices. The optical pick up unit 103 is used to reproduce data stored in the optical recording medium 101, or is used as a signal sensor or control device for recording data transmitted from the host 113 onto the optical recording medium 101.
The AFE unit 120 records data in the optical recording medium 101, and then converts the data recorded in the optical recording medium 101 into radio frequency (RF) data signals.
The DSP 107 converts the RF signals into digital signals. The decoder 109 encodes the digital signals in such a manner that the error rate of the converted data is minimized. The host I/F 111 functions as an interface between the host 113 and the decoder 109. The host 113 may be, for example, a personal computer (PC).
The reproduction operation of the optical recording/reproducing apparatus 100 may proceed as follows.
First, the optical pickup unit 103 irradiates light onto the optical recording medium 101, and light that is reflected is received by the optical pickup unit 103. RF signals are then generated by reading mark and space patterns on the optical recording medium 101 by using a diffraction optics phenomenon. The AFE unit 120 amplifies and standardizes the RF signals. By passing through the DSP 107, the RF signals are converted into square wave signals. By passing through the decoder 109, the RF signals are encoded and are converted into data that can be recognized by the host 113.
The AFE unit 120 converts the data read from the optical recording medium 101 into RF signals. The data read from the optical recording medium 101 is converted into RF signals by passing the data through a filter that is included in the AFE unit 120. As the RF signals pass through the filter, a time delay occurs. In other words, after passing through the filter, an input signal is output such that the waveform of the input signal is delayed a little on a time axis, as if having a phase shift. The time delay that is caused by passing a signal through the filter is called group delay.
In addition to delaying a signal, group delay may also distort the signal. If the group delay is large, the phase of the frequency band through which signals pass may also be distorted. Irregular phase shift in the frequency band causes signal distortion. For example, when group delay is generated in the filter, an input RF signal having a mark length of 5 T may be output as a signal having a mark length of 4 T. Such a signal distortion may result in incorrect data being output from the optical recording/reproducing apparatus 100.